By default, Joomla backend login URL is www.yoursite.com/administrator. It’s easy to remember but bad from security point of view.
If you change administrator login to www.yoursite.com/administrator/?secreturl , your site will be more protected.
For this, I use free Joomla plugin kSecure (for Joomla 1.5 .1.7, 2.5, 3.x)- downloading requires registration, but it’s free and plugin is great! Another plugin is OSE Secure. Best part is that visiting www.yoursite.com/administrator/ would redirect to www.yoursite.com.